THYSANOCRINID^. 203 



Idiocrinus elongatus W. and Sp. 



Plate XYIII. Figs. 8a to c. 



1892. W. and Sp., Am. Geologist, Vol. X. (September), p. 136. 



Syn. Gazacrinus inornatus S. A. Miller ; Adv. Sheets 18th Rep. Geol. Surv. Indiana, p. 49, Plate 

 5, Pigs. 9, ]0, 15, 16, 17 (published October 26), 1892. 



A small species. Calyx obconical ; the ventral disk almost on a level 

 with the upper margins of the dorsal cup ; the cup deeply excavated at the 

 bottom, the basals forming a large funnel-shaped pit. Plates without orna- 

 mentation and flat, except the radials, which are a little convex, and rise 

 slightly above the plane of the cup. Suture lines not grooved. 



Infrabasals minute, constituting the bottom of the basal concavity. Basals 

 extremely large and elongate, the lower end curving abruptly inward, and 

 forming a sharp edge around the bottom of the calyx ; the exposed part of 

 the plates rising to more than one third the length of the dorsal cup. Radi- 

 als once and a half as wide as long, distinctly angular at their lower faces ; 

 the two posterior plates hexagonal, being distinctly truncated by the anal 

 plate. Costals together less than half the size of the radials; the first linear; 

 the second a very little longer, and its upper angles quite obtuse. Dis- 

 tichals two, somewhat higher and wider than the costals; the upper semi- 

 free. Interbrachials one, those of the four regular sides resting upon the 

 deeply sloping sides of the radials, the anal one upon the narrowly truncated 

 basal ; all extending to the upper end of the calyx, and all longer than wide. 

 Arms apparently ten, their dorsal faces flat. 



Ventral disk but slightly elevated, consisting of ^\m^ large, oblong, trian- 

 gular plates, one to each area, which do not meet in the centre, but leave 

 a moderately large, pentalobate open space at the summit. The sides at the 

 lower ends of the plates to two thirds their length are not in contact, but 

 leave elongate spaces, which were apparently occupied by the ambulacra; 

 the lateral edges are slightly serrated, and bend outward for the reception of 

 the covering pieces. Near the summit, the plates meet in a similar manner 

 as the deltoids of the Blastoid genus Pentremites, by means of lateral pro- 

 cesses, and these are converted into grooves which communicate with the 

 vacant space in the centre. The structure is such as to indicate that the 

 ambulacra followed these grooves before entering the peristome, and that 

 the centre was covered by an oral pyramid, as in the allied Idiocrinus ventri- 

 cosuSy in which the pyramid was found in position. The posterior inter- 



